Digestion of nutrients begins in your mouth with chewing and enzymatic action on carbohydrates. After passing through the esophagus, carbohydrates and proteins undergo further breakdown in your stomach. Here, mechanical, chemical and enzymatic processes turn nutrients into a liquid called chyme, which pushes slowly into the small intestine. After complete breakdown in the small intestine, nutrient molecules are absorbed into your bloodstream through structures called villi.
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Small Intestine Anatomy
According the National Digestive Diseases Information Clearinghouse, your small intestine is the major digestive organ. Essentially a tube 20 to 21 feet long, it contains three sections -- duodenum, jejunum and ileum. Up to 5 million tiny finger-like projections called villi cover the inner surface of the intestine and increase by five times the surface area exposed to intestinal liquid. The outer surfaces of villi consist of thousands of absorptive cells covered with microscopic microvilli that further enhance absorptive capacity.
The primary purpose of your duodenum is to receive the stomach fluid, a mixture which contains highly acidic water, minerals, carbohydrate and protein particles, and undigested fat. Enzymes and buffer salts from your liver and pancreas neutralize the acidity and further digest carbohydrates and proteins. Bile from your liver allows fat to disperse into the watery fluid and come into contact with fat-degrading enzymes. The intestinal lining also contains glands that secrete digestive enzymes. Folds of tissue on the duodenal surface spiral the fluid towards the jejunum like a slow drain pipe and facilitate mixing with the enzymes.
Most nutritional absorption takes place in your jejunum, according to the Short Bowel Syndrome Foundation. Polysaccharides, amino acids, water-soluble vitamins, minerals and bile-connected fatty acids with fat-soluble vitamins enter absorptive cells by diffusion or transport mechanisms. The bloodstream absorbs the water-soluble components, while the lymphatic vessels absorb the fatty-bile molecules. After traveling to the liver for further processing and detoxification, the nutrients pass to other cells for energy production.
The ileum is the longest part of your small intestine. Most nutrients have been absorbed by the time the diluted chyme reaches the ileum. But vitamin B-12 travels though your duodenum and jejunum to reach the ileum before absorption. Water and electrolytes enter the bloodstream from the lower part of the small intestine. At the end of the ileum lies a one-way valve that prevents chyme from leaking back in once it has entered the bacteria-laden large intestine.